What daylight factor does a warehouse need?

Daylight factor is the ratio of the light indoors to the light outdoors at the same moment, written as a percentage. Stand at a point on the warehouse floor, measure the illuminance there, measure the illuminance outside under an open sky at the same instant, divide one by the other and multiply by a hundred. A daylight factor of 2 per cent means the floor is receiving two hundredths of whatever the sky is giving.

The measurement is taken under a standard overcast sky, the dull grey one, on purpose. That removes the sun and the season from the number, so the figure describes the building rather than the weather. It is a worst-case daylight figure. On a bright day the floor gets far more, but the overcast case is the one that decides whether you can trust the space to be usable without the lights on when the cloud rolls in.

Two figures matter: the average across the whole floor and the uniformity, which is the ratio of the darkest point to the average. A shed can hit a healthy average and still have gloomy aisles between bright pools if the rooflights are badly spaced. For storage and movement you want the light spread evenly, so uniformity is worth as much attention as the headline percentage.

A warehouse is rarely one job. The lighting a forklift driver needs to move pallets safely is different from the lighting a packer needs to read a barcode, and the daylight factor should follow the task rather than a single blanket figure.

As a rough guide to what these mean, below 2 per cent a space feels dim and leans on artificial light all day. Around 2 per cent it reads as a daylit room. At 5 per cent and above it feels bright and airy, and the lights can stay off through most daylight hours. The BREEAM and CIBSE guidance that assessors work to sits on the same scale, so specifying to these numbers is also the language a design team will recognise.

In a single-storey shed the daylight comes down through the roof, so the daylight factor is driven almost entirely by rooflight area, how the units are spread out and how much light the glazing lets through. Walls and the odd window barely register more than a few metres in.

As a working rule, glazing about 10 per cent of the roof area with diffusing rooflights gets a typical warehouse to roughly a 2 per cent average. Lift that towards 15 to 20 per cent and you move into the 3 to 5 per cent band for the working zones. Spread the units evenly across the roof rather than clustering them, because a run of rooflights over one bay and a bare roof over the next gives you a good average and poor uniformity, which is the worst of both.

Diffusing glazing earns its place here. A clear rooflight throws a sharp bright patch and a hard shadow that tracks across the floor through the day. A diffusing unit spreads the same light into a soft, even wash, which is what evens out the uniformity and stops direct sun falling across a picking screen or a driver’s eyeline. There is a ceiling to bear in mind too: Building Regulations Part L limits how much rooflight area you can add before it starts to count against the building’s thermal performance, so more glass is not automatically better and the specification has to balance daylight against heat loss.

Every rooflight that lets daylight in also lets solar heat in, and on a large industrial roof that can add up fast. A warehouse roof is close to horizontal, so it faces the high summer sun almost square on and collects heat from a huge glazed area no matter which way the building is turned. Chase a high daylight factor with clear glazing and you can build a very well lit space that overheats badly by mid-afternoon, and a warm warehouse is uncomfortable to work in and expensive to cool.

This is where the glass specification does the real work. The G-value is the share of the sun’s heat that gets through the glazing, and solar-control glass holds that figure down while still letting useful daylight through. On a south-facing site like the estuary here in Southend, where an open southern horizon and light bouncing off the water add to the load, that balance matters even more, and it is the same reasoning behind our energy-efficient skylight installation work. Diffusing units help with glare, which is the other half of the problem: bright rooflights reflecting off a screen or a shrink-wrapped pallet make close work harder, not easier. The aim is a steady, even daylight factor without the heat and the glare that come free with cheap glass.

Start with the tasks, not the roof. Walk the building and note what happens where: bulk racking, circulation routes, goods in, the pack benches, any inspection bays. Set a target daylight factor for each from the table above, then work back to how much rooflight area, spread how evenly, gets you there. For most sheds that lands somewhere around 10 to 20 per cent of the roof glazed, tuned up or down for the daylight the working zones actually need.

Then decide the glass before anyone talks about units. On a flat or near-flat industrial roof the heat and glare question is not optional, and diffusing solar-control glazing is usually the right answer for keeping the space both bright and workable. Our commercial skylight installation page sets out how that comes together across a larger roof, and the same orientation-first thinking runs through our flat roof skylight installation work.

If you want a hand turning a daylight-factor target into a rooflight layout and a glass specification, tell us the roof size, the pitch and what happens on the floor below. Ring 01702 898232 or request a quote, and we will work back from the light you need rather than the number of holes in the roof.